By way of an example of the problems with which the invention is concerned, a standard diffusion pumping system has a gate valve between the main chamber to be evacuated and a cold trap and a water cooled cold trap between the cold trap and the diffusion pump which is backed by a mechanical vacuum pump.
A gate valve has large areas to be exhausted and typically includes castings which are costly to machine and if not machined present rough surfaces interfering with pumping efficiency.